Linear motors having stationary armatures containing coils and movable stages containing magnets are well known in the art. Also known are linear motors having stationary magnets and moving coils. One type of such linear motors is disclosed in U.S. Pat. No. 4,749,921 to Chitayat. The linear motor disclosed in this patent has a series of armature windings mounted to a base plate and a stage having a series of magnets that is free to move on the base plate. The stage is urged in the desired direction by applying AC or DC excitation to the coils. When such a linear motor is used in a positioning system, the relationship between the location of the stage and locations of the coils must be accounted for. In another linear motor, commutator contacts are pendant from the stage. The contacts contact one or more power rails, and one or more coil contacts. As the stage moves along the armature, the location of the stage, relative to the armature is automatically accounted for by applying power to the stationary armature windings through the commutator contacts. In yet other linear motors, it is conventional to employ a service loop of wires between the moving stage and the stationary elements.
Typically, the location of the stage is updated using a magnetic or optical position encoder on the stage which senses markings on an encoder tape stationary alongside the path of the stage. The location is transferred by the service loop to a stationary motor controller. Generally, the important location information is the phase of the stage relative to the phase of the armature. For example, in a three-phase armature, the windings are disposed in repeating sets of three for phases U, V and W. If the center of the U phase winding is arbitrarily defined as 0 degrees, then the centers of the V and W windings are defined as 120 and 240 degrees. There may be two, three or more sets of windings as required for the travel distance of the stage. Normally, all U phase windings are connected in parallel. The same is true of all V and W phase windings. Thus, when the location of the stage requires a certain voltage configuration on the particular windings within the influence of the magnets on the stage, besides powering these windings, all of the other windings in the armature are also powered. The maximum force obtainable from a linear motor is limited by the allowable temperature rise in the armature windings. When all windings are powered, whether they contribute to motor force or not, more armature heating occurs than is strictly necessary for performing the motor functions. Some linear motors in the prior art have responded to this heating problem using switches that are closed only to the armature windings actually within the influence of the magnets.
For reference, FIG. 1 is a side view of a linear motor 100 in accordance with the prior art. The linear motor 100 includes a stage (or mover) 110 and a stator 120. The stage (here, the armature) 110 includes coils 130 and the stator (here, the field) 120 includes magnets 140. The linear motor 100 is controlled by an external driver/controller 150 that is connected to the linear motor 100 by umbilical wires 190. The umbilical wires 190 include: three wires for U, V, and W signals 160 from the stage 110; five wires for power, ground, and U, V and W signals from the Hall Effect sensor 170; and, five wires for power, ground, and A, B and Z signals from the position sensor 180 on the stage 110. The Hall Effect sensor 170 is used for detecting magnetic poles for commutation purposes.
Now, linear motors are increasingly being employed in manufacturing equipment. In such equipment, nominal increases in the speed of operation translate into significant savings in the cost of production. It is particularly desirable to produce as much force and acceleration as possible in a given linear motor. An increase in force generated requires either an increase in magnetic field intensity or an increase in current applied to coils of the armature. In a permanent magnet linear motor, the available magnetic field intensity is limited by the field strength of available motor magnets. Power dissipated in the coils increases at a rate equal the square of the current. Attendant heat generation limits the force that may be achieved without exceeding the maximum armature temperature. Therefore, improvements in the power dissipation capacity of linear motors provide for increases in their utility.
In typical manufacturing equipment, a linear motor may be employed for driving a positioning table along an axis. For example, positioning tables are commonly used for moving a work object such as an electronic device in a precise path for performing an operation or inspection on the work object. Desirable characteristics of such positioning tables include precision, compactness, the maximum speed at which the table can be driven and the accuracy with which the table may be positioned. U.S. Pat. No. 4,151,447 to von der Heide, et al., discloses a linear DC motor having rows of pairs of vertically standing permanent magnets between which flat coils are arranged to travel. The polarity of DC power to the flat coils is switched by a magnetic field or electro-optical sensor at predetermined points in the travel of the flat coils. The apparatus in this patent employs trailing cables for feeding power to the coils.
U.S. Pat. No. 4,761,573 to Chitayat discloses a linear DC motor suitable for driving a positioning table. This linear DC motor includes a linear toothed structure including coils wound around the individual teeth to form a repeating line pattern of electrically produced magnetic poles facing a corresponding parallel array of magnets arranged with alternating magnetic polarity having their broad faces closest to the toothed assembly. A brush assembly is provided on the movable element for contacting a linear slip ring assembly on the stationary element for switching the polarity of voltage applied to energizing coils of the motor. Linear power pickup rails are used in conjunction with brushes and linear slip rings for feeding and controlling power to energizing coils. Furthermore, a brush and power pickup brush assembly is disclosed for feeding first and second electrical polarities to energizing coils which employs two identical comb-like structures for both picking up power from linear power pickup rails and for feeding power to the coils through a linear slip ring.
Another brush and rail power pick-up arrangement is disclosed in U.S. Pat. No. 4,789,815 to Kobayashi, et al. This patent discloses a movable stage having control and driver means for supplying electric power to coils in the movable stage. The electric power is delivered to the control and driver means through brushes which make contact with rails mounted on the frame. The direction and position of the movable stage are controlled through the supply of power to the rails (i.e. on, off, and polarity). The linear motor thus disclosed is directed toward the control of curtains in vehicles.
Thus, there is a growing commercial use of high performance, linear motors in various manufacturing and other applications. One recognized disadvantage of prior art linear motors is the cumbersome umbilical wires that connect the moving armature or stage to the controller and power source. For example, the umbilical for a prior art three-phase, brushless motor may have three power lines, five signal lines for the armature commutating signals, and eight signal lines for armature position signals. The need for a cable loop connecting moving and stationary elements is inconvenient and limits the flexibility with which a system can be designed. The wiring harness requires additional clearance from the linear motor to prevent entanglement between the motor and any equipment or items that may be adjacent to the linear motor path. In addition, the wiring harness adds additional weight to the moving element of the linear motor. Furthermore, manufacturing of a linear motor employing a wiring harness incurs additional cost of material and assembly labour. Therefore, it would be desirable to eliminate the use of a wiring harness in a linear motor to decrease the cost of assembly, decrease the overall weight of the moving element, and to eliminate the clearance restrictions on the linear motor's utility. Another recognized disadvantage is the need to remove heat from the moving stage (i.e. armature). Where a coolant is used, the umbilical includes, in addition to the wires, a tube to carry the coolant to a coolant coil embedded in the armature and a tube to carry the coolant from the coil. The result is a heavy, cumbersome, umbilical of wires and tubes, festooned along the path over which the stage moves.
To overcome some of these disadvantages, wireless or semi-wireless linear motors have been developed and have been disclosed, for example, in U.S. Pat. Nos. 5,936,319 to Chitayat and 6,005,310 to Mosciatti, et al. U.S. Pat. No. 5,936,319 discloses a communications device on a movable stage which wirelessly informs a motor controller about the position and/or incremental motion of the movable stage. The movable stage includes a position encoder and any wireless transmission system may be used including radio and infrared. However, the movable stage includes permanent magnets while the fixed path includes armature coils. U.S. Pat. No. 6,005,310 discloses a movable stage with a wireless transmitter (e.g. radio frequency or infrared) for transmitting commutating and position signals to an external motor controller. However, while the movable stage includes coils, the motor controller is connected to the movable stage by an umbilical cord.
A need therefore exists for an improved wireless linear motor which overcomes at least some of the drawbacks of the prior art. Accordingly, it is an object of the present invention to provide such a linear motor.